l. Field of the Invention
The present invention relates to an optical disk reproducing method for an optical disk employing the V-shaped groove system and also to an optical disk reproducing apparatus employing a light detecting apparatus particularly designed therefore.
2. Description of the Prior Art
To increase the density of data recorded in optical disks, the V-shaped groove system has been proposed which is disclosed, for instance, in U.S. Pat. Nos. 4,310,916 (corresponding to Japanese Patent Laid-open Publication No. SHO 56-58144), 4,569,038 (corresponding to Japanese Patent Laid-open Publication No. SHO 57-105828) and 4,534,021 (corresponding to Japanese Patent Laid-open Publication No. SHO 58-102339).
FIG. 1 is an enlarged perspective view of a replica section of a V-shaped groove disk, which includes a transparent substrate 1, V-shaped grooves 2, and signal pits 3 formed on the slopes thereof. The laser for reproducing signals is irradiated from the bottom side of the transparent substrate 1, as shown by an arrow, and reflected to the side of the transparent substrate 1 by a reflection film (not shown) formed on the surface of the v-shaped grooves of the substrate 1.
The optical system for signal reproducing will be briefly described with reference to Fig. 2. The light from a semiconductor laser 4 passes through a diffraction grating 5, and a half mirror 6, collimated by a collimating lens 7, and focused on the V-grooved disk by an object lens 8. The light reflected from the disk passes again through the object lens 8 and the collimator lens 7, and after reflection by the half mirror 6, ixradiated onto a light detector 10, with astigmatism being given by a cylindrical lens 9, such as a biconcave cylindrical lens. In Fig. 2, the axis of the cylindrical lens g is parallel to the V-shaped grooves (perpendicular to the paper face). Based on the signal obtained from the light detector 10 by the irxadiation of reflected light, the position control of the laser spot on the disk and the reproduction of the recorded signal are effected.
Furthermore, the laser beam of the semiconductor laser is practically split into three bundles of beams by the diffraction of the diffraction grating 5 so as to form 3 spots on the v-shaped disk as shown in Fig. 3. In the reproduction optical system of Fig. 2, only one beam of the three beam is shown for simplicity of the drawing.
Among the three spots on the disk shown in Fig. 3, the middle spot 11 is focused onto the xidge or valley of the V-shaped groove so as to effect the focusing control and tracking control, while the spots 12 and 13 of the +1st order and -1st order diffraction light are focused onto the adjacent slopes of the V-shaped groove for the signal reproduction. Since the two signals on the slope can be independently reproduced, the transfer rate becomes twice.
The light detector 10 includes three detection units U1, U2 and U3, as shown in Fig. 4, at locations corresponding to three laser spots formed on the light detector 10. Each of the three detection units is divided into two or more segments. The dividing of the unit into a number of segments is disclosed, for example, in Japanese Patent Laid-open Publication SHO 60-212836 in which it is disclosed that the three units Ul, U2 and U3 are divided into 8 segments, such as shown by 14 to 21 in FIG. 4.
The three bundles of beams reflected from the V-shaped groove impinge on the light detector 10 as shown in FIG. 4. Since the axis of the cylindrical lens 9 is parallel to the V-shaped groove, a line dividing between the detection segments 18 and 19 or a line dividing between the detection segments 20 and 21 coincides with the direction parallel to the V-shaped groove. The spot position control is effected by the reflection beam lla, while the focusing control of astigmatism system is effected by the signal obtained by subtracting the sum signal of the detection segments 16 and 17 from the sum signal of the detection segments 14 and 15. The tracking control of push-pull system is effected by the signal obtained by subtracting the signal of the detection segment 17 from that of the detection segment 16. The signals on the adjacent slopes can be reproduced by reflection beams 12a and 13a. For the signal reproduction, the ccmbination of detection segments 19 and 20 is used, and the siqnal on one slope is reproduced by the detection segment l9, the signal on the confronting slope is reproduced by the detection segment 20. In same optical systems, the combination of the detection segments 18 and 21 is used for the signal reproduction.
The construction of the reproduction optical system in FIG. 2 is general for the reproduction of the optical disk, and used or not only the V-shape disk but also for the conventional flat plate disk such as CD and video disk. However, the division of detection segments of the light detector and roles of respective detection segments are different.
In the conventional flat plate disk, although three spots 11 to 13 are arranged as shown in FIG. 5, a light detector 10 as shown in Fig. 6 is used for the reproduction of the flat plate disk. The detection segments 18 and 19 in FIG. 4 are integrated as the detection segment 22, while the detection segments 20 and 21 are also integrated here as the detection segment 23, thus constituting a light detector divided into 6 portions on the whole.
FIG. 6 shows three reflection beams from the disk. The focuslng control of the spot position and the signal reproduction are effected by the reflection beam 11a, while the focus control of astigmatism system is effected by the signal obtained by subtracting the sum signal of the detection segments 16 and 17 from the sum signal of the detection segments 14 and 15, and the signal reproduction is effected by the sum signal of the detection segments 14, 15, 16 and 17. Furthermore, light reflected respectively from 12a and 13a are irradiated onto the detectlon segments 22 and 23, and by comparing the received light amount thereof, the tracking control of three beam system is effected. Thus, the reproduction in the case of the flat plate disk is conducted.
Now, in the v-shaped groove system, for raising the record density, it is desired to decrease the track pitch (center line distance between adjacent slopes) by narrowing the distance between v-shaped groove ridges. Even at this time, it is necessary to reduce the leakage signal (cross-talk) between adjacent slopes, and to this end, there have been proposed several reproduction methods for the V-shaped disk. The typical three methods therefor will be described below.
First, in U.S. Pat. No.4,310,916, it is proposed to irradiate laser beam onto the slope perpendicularly thereto, and all of the reflected light is received. In an object lens of more than 0.5 high NA (numerical aperture), the incident angle of laser beam is to be set within 1 to 2 degree to the normal of the optical axis of the object lens and to the normal of the surface of the disk substrate (transparent substrate 1 in FIG. 1), and with a larger incident angle, aberration will take place and thereby the laser beam can not be sufficiently focused, resulting in the deterioration of the reproduced signal quality. In this conventional example, the slope angle of the V-shaped groove slope is more than 5 degree at the smallest, and therefore, the incident angle of laser beam relative to the surface of the transparent substrate in FIG. 1 exceeds the allowable range, and it is very difficult to put the conventional method into practical use.
Secondly, the reproduction method proposed by U.S. Pat. No. 4,569,038 will be described. This method is arranged to make laser beam incident upon the disk approximately parallel to the optical axis of the object lens and also approximately parallel to the normal line of the surface of the transparent substrate 1 of FIG. 1, thereby capable of focusing the laser spot on the V-shaped groove with small aberration. Here, the term "approximately parallel" means that the angle between two lines is within one degree. Since the V-shaped groove slope is inclined, the direction of the reflected light departs from the optical axis of the object lens. Since, on the reproducing slope of the V-shaped groove, the center of the laser spot is tracked, most of the light is irradiated onto the reproducing slope, and most of the ref1ected light returns also onto a semicircular or a half-circular portion of the object lens. Furthermore, although the reflected light from the adjacent slope returns also onto the opposing half-circular portion of the object lens, since only the peripheral portion of the laser spot is irradiated on the adjacent slope, the ref lected light amount is small.
More specifically, the reflected light from the laser spot on the slope of V-shaped groove 2 is directed mostly toward the semicircular portion of the object lens 8, as shown in Fig. 7a, so that the distribution of the reflected light becomes such a curve D as shown in Fig. 7b. This reflected light distribution D is an example in which laser beam is irradiated onto a position where no signal pit is present on the slope, and divided into a reflected light portion of large intensity and a reflected light portion of small intensity. This larger reflected light portion corresponds to the reflected light from the reproducing slope, spreading to more than half of the object lens .
When there is a signal pit on the reproducing slope, respective mountain portions in the distribution D in Fig. 7b are decreased, with the decreased amount being spread on the periphery, which is not shown in FIG. 7b. Further more, when there is a signal pit on the adjacent slope, the smaller mountain portion in the distribution D in FIG. 7b is decreased, with the decreased amount being spread onto the periphery, so as to constitute the leakage signal (cross-talk) from the adjacent slope.
Although the reflected light distribution D in FIG. 7b is reduced in its beam diameter by the lens system in the reflection path, the spot pattern irradiated on the light detector 10 will have a distribution similar to that shown in the FIG. 7b. In U.S. Pat. No. 4,569,038, it is analyzed that the cross-talk of the reproduced signal can be reduced by the interference effect of the reflected light at a region X which is smaller than a half-circular portion. The light detector used for reproducing data obtained at such region X can be accomplished by inserting a non-sensitive segment between detection segments 18 and 19 and between detection segments 20 and 21 in FIG. 4, as is proposed by Japanese Patent Laid-Open Publication No. SHO 60-212836.
The above is the outline of the disclosures in U.S. Pat. No. 4,569,038 and Japanese Patent Laid-open Publication SHO 60-212836.
Thirdly, in the method proposed by U.S. Pat. No. 4,534,021, laser beam is also made incident in parallel to the optical axis of the object lens, and most of the reflected light is condensed onto a half portion of the lens. In this patent, although the reflected light passing through the object lens and other reflected light from outside the object lens are received, with respect to the reflected light passing the object lens, the reflected light passing through the half-circular portion is received, such as shown by region Y in FIG. 7b. Thus, it may be possible that the cross-talk can be made small to some extent, but it is not to an optimum level.
In the analysis of the above conventional examples, an assumption that laser light is always in complete coherence is included.
When no signal pit is form on the slope, and a thin film of phase change material is formed on the surface of the V-shape groove and only the reflectivity of the recording portion is changed by the signal record, there is no surface ruggedness on the slope and the assumption of coherence is realized to some extent, and the analysis result may explain the actual experimental result. Furthexmore, the same will apply in the case where a film of photo-magnetic material is formed on the V-shape groove surface, and the magnetization direction is changed by the signal record.
However, in the case of a reproducing disk where a signal pit is formed on the V-shaped groove slope, since it is difficult to form a signal pit on the disk in an ideal shape, and the bottom surface and the periphery may be somewhat disturbed, the reflected light is not a completely coherent light, with the actual phenomenon being considerably different from those shown in the above conventional examples. Therefore, in the above conventional examples, it is impossible to effect an optimum signal reproduction.
For example, many components disturbed by the scattering are included in the reflected light, and coherence is partially broken, resulting in reduction of the interference capability. Scattered light has no specific directionality, and these cross-talk components arising from the scattering are spread over the entire reflected light so as to be included almost homogeneously. Therefore, the tendency that the cross-talk becomes a minimum at a particular portion of the reflected light is reduced, and in the case of receiving only one portion of the reflected light distribution, the reproduced signal does not become a maximum, and it is disadvantageous for obtaining a signal of high quality.
Therefore, as an actual problem, it is necessary to provide an optimum reproducing method for the V-shaped groove disk which is formed with signal pits on the slope thereof.